The present invention relates to an apparatus and a method for maintaining braking force acting on the wheels of a vehicle that has been stopped through depression of a brake pedal even after the brake pedal is released from depression.
Generally, if the brake pedal of a vehicle is released from depression when the vehicle is stopped on an inclined surface such as a slope, braking force acting on each of the wheels quickly drops, causing undesired downward movement (sliding) of the vehicle along the inclined surface. This may prevent the driver from smoothly re-starting the vehicle. To avoid this, an apparatus and a method for maintaining braking force of a vehicle involving hill-hold control have been proposed (as described in, for example, Japanese Laid-Open Patent Publication No. 10-24817). Through the hill-hold control, undesired movement of the vehicle is suppressed by maintaining braking force acting on the wheels of a stopped vehicle even after the brake pedal is released from depression.
Specifically, as illustrated in FIG. 8, in the braking force maintaining apparatus of a vehicle described in Japanese Laid-Open Patent Publication No. 10-24817, a RAM memorizes brake fluid pressure of each of wheel cylinders (braking means) at reference time T10 as reference brake fluid pressure BBP10, after the vehicle is stopped through depression of the brake pedal. A predetermined reference value is added to the reference brake fluid pressure BBP10 to obtain a threshold value KBP10. The threshold value KBP10 is stored in the RAM. Afterwards, if the braking pedal is further depressed and the brake fluid pressure in the hydraulic circuit becomes greater than or equal to the threshold value KBP10, the fluid pressure in each wheel cylinder is maintained so as to maintain the braking force applied to the wheels by the wheel cylinders.
However, since the threshold value is set based on the brake fluid pressure in each wheel cylinder at a reference time after the vehicle is stopped, there may be cases where hill-hold control cannot be initiated regardless of further depression of the brake pedal. In other words, as illustrated in FIG. 8, if the reference time is set to a relatively great value, or a reference time T11, the brake fluid pressure in each wheel cylinder at the reference time T11 (a reference brake fluid pressure BBP11) becomes relatively high. This correspondingly increases the resulting threshold value KBP11. In this case, regardless of further depression of the brake pedal intended to initiate the hill-hold control, the brake fluid pressure of each wheel cylinder may remain smaller than the threshold value KBP11. The hill-hold control thus cannot be initiated.